The present invention relates in general to bus termination circuits and, more particularly, to a switchable active bus termination circuit operating serially with the electrical bus.
Most computer systems, for example in office environments, communicate with peripheral devices over an electrical conduction bus or multi-conductor cable. Computers and computer peripheral devices, such as printers, modems and disk drives, are typically networked together allowing communication between the host computer and peripherals via the electrical bus. One example of an electrical bus is the small computer system interface (SCSI) bus approved by the American National Standards Institute (ANSI).
The electrical bus is susceptible to reflections of the logic signals along the transmission line unless properly terminated with an impedance equal to the characteristic impedance of the electrical bus. In prior art applications, the termination is typically a passive two-resistor divider network (220 ohms and 330 ohms) coupled between a positive power supply conductor (5 volts) and ground potential. In an AC analysis, the resistors operate in parallel and provide an effective impedance of 132 ohms to ground to closely match the characteristic impedance of the electrical bus. In a DC analysis, the resistor divider produces a reference potential, approximately 2.85 volts, at the interconnection of the resistors for establishing the output high voltage level (V.sub.OH). The SCSI standard requires the peripheral devices to operate with open-collector outputs such that the electrical bus is normally high and active low.
One problem with the passive resistor termination is the inherent drift and inaccuracies of setting resistor values within an integrated circuit resistor array. Since the termination resistors set the V.sub.OH level and determine the sink current through the open-collector devices in the computer peripherals, the value of the resistors must be carefully selected. Resistor tolerance is also a problem for thin film processing because the sheet resistance varies with manufacturing process, and with temperature (luring operation.
Another problem is the constant DC power consumption between the positive power supply conductor through 550 (220+330) ohms of resistance to ground. Each electrical conduction bus terminator dissipates approximately 45 milliwatts.
Presently, most SCSI systems use a terminating resistor array in a single in-line package (SIP) which can be plugged into a socket of the terminating peripheral device. To connect the peripheral to the end of the network, a resistor SIP is plugged into a socket thereby designating the peripheral as the last device on the bus. If the peripheral device is not the last device on the bus, the resistor SIP must be removed. The manual operation of inserting and removing the resistor SIP in the peripheral device is inconvenient and often confusing especially for nontechnical users.
Therefore, a need exists for a switchable bus terminator circuit that can be permanently installed in computer peripheral devices and activated by a control signal.